When molecules go through a mass spectrometer, some of
them arrive intact at the detector, but many of them break into pieces in a
variety of different ways. To establish a charge on a molecule, an electron had
to be removed; removal of that electron is effected through a collision, usually
with a high-energy electron. During that collision, energy is transferred from
the high-energy electron to the molecule, and that energy has to go somewhere.
Part of it gets partitioned into various bond vibrations, so bonds start to
vibrate quite a lot, until some of them snap completely. The molecular ion
breaks apart and forms a fragment ion.

Some fragment ions are very common in mass
spectrometry. These ions are seen frequently for either of two reasons:

there is not a pathway available to break the ion down.

the ion is relatively stable, so it forms easily.

Fragmentations occur through well-defined pathways or
mechanisms. A mechanism is a step-by-step series of events that happens in a
reaction. It is important to understand how reactions happen, but we will look
at fragmentations when we study radical reactions.

However, it is useful to know what factors make
cations stable.

Some Common Ions

There are a number of ions commonly seen in mass
spectrometry that tell you a little bit about the structure. Just like with
anions, there are a couple of common factors influence cation stability:

Electronegativity plays a role. More electronegative atoms are less
likely to be cations.

Polarizability also plays a role. More polarizable atoms are more likely
to be cations.

However, in most cases, we will be looking at carbon
with a positive charge, and there are additional factors to distinguish between
them

Delocalization stabilizes a cation by spreading out the charge onto two
or more different atoms.

In Lewis structure terms, the easiest way to delocalize charge is via
resonance.

Resonance can involve other carbons, like in allyl and benzyl cations.

Resonance can also involve other atoms, like in acylium or iminium cations.

Delocalization can also be accomplished through inductive effects. The
trend in carbocations is that the more substituents on teh carbocation, the
greater the stability.

Tertiary cations, with three substituents on the carbocation, are more
stable than secondary cations, with two substituents on the carbocation.
Secondary cations are more stable than primary ones. Primary cations are
more stable than methyl cations.

Molecular orbital calculations suggest that the cation
is stabilized through interaction with neighboring C-H bonds in the alkyl
groups. Specifically, a C-H sigma bonding orbital has symmetry similar to the
empty p orbital on the positive carbon. The lobes on the two orbitals can
overlap such that they are in phase, and that allows electrons to be donated
from the C-H bond to the central, electron-deficient carbon. Formally, there is
a bonding interaction and an antibonding interaction between these two orbitals.
Since one of these orbitals is empty, the antibonding combination remains
unoccupied. The bonding combination is populated, however, and since it is lower
in energy than either the p orbital or the C-H sigma bond (all bonding
combinations are lower in energy than the orbitals that combine to form them),
there is a net decrease in energy.

Problem MS10.1. Draw as many resonance
structures as you can that help explain teh stability of the following cations: